Fitness of geonegative flies from unselected populations of D. melanogaster have been found to exhibit much greater fitness than geoneutral or geopositive flies. On the otherhand no fitness differences have been found among photopositive, photoneutral or photonegative D. melanogaster from unselected populations. However, by rearing D. pseudoobscura under complex environmental conditions, we have been unable to increase the heritability of phototactic behavior, which suggests that natural selection does act to maintain genetic variation for this trait. Genetic analysis has shown that photonegative behavior is sex linked in D. melanogaster but autosomally inherited in its sibling species D. simulans. Inheritance of geopositive and geonegative behavior has been shown to be sex linked in two more populations of D. pseudoobscura. Detailed genetic analysis of geotaxis in D. pseudoobscura is now in progress. Variables such as age, temperature, and humidity are being found to exert important effects upon phototactic behavior in D. melanogaster and other species. These studies are being expanded to include phototactic behaviors in larvae as well. BIBLIOGRAPHIC REFERENCES: Markow, T.A. and Scavarda, N.J. 1976. Effect of age and of screening pigment mutants on the phototactic behavior of Drosophila melanogaster. BEHAVIOR GENETICS (in press). Markow, T.A. 1975a. A genetic analysis of phototaxic behavior in Drosophila melanogaster. I. Selection in the presence of inversions GENETICS 79: 527-534.